1. Field of Use
This invention broadly relates to solid state integrated current and voltage reference sources which are independent of supply line voltages. More particularly, this invention relates to a stabilized current or a stabilized voltage reference source where the provided current or voltage is both temperature compensated and independent of supply line voltage changes.
2. Background Art
In providing a current or voltage source, it is desirable that the output current or voltage vary as little as possible regardless of the change in temperature or supply voltage. It is also desirable to avoid the use of PNP transistors in the circuit as the fabrication of precision PNP transistors has proved difficult. With the foregoing in mind, various current and voltage sources have been proposed.
A prior art voltage source which is substantially temperature independent is seen in FIG. 1. The circuit of FIG. 1 basically comprises an amplifier, two transistors QA1 and QB1, and two resistors RA1 and RB1. In reviewing the operation of the circuit of FIG. 1, it is important to recall that the base-to-emitter voltage (V.sub.be) of an NPN transistor is given approximately as: EQU V.sub.be =(kT/G) ln(I.sub.c /I.sub.s) (1)
where k is Boltzmann's constant, g is the electric charge, T is the absolute temperature (kT/g sometimes being referenced as V.sub.T), I.sub.c is the collector current, and I.sub.s is the transistor saturation current which is proportional to the emitter area (or "width"). Since the amplifier of FIG. 1 causes the currents I.sub.CA and I.sub.CB to be nearly equal, upon balancing the voltages, and in accord with equation (1), I.sub.CB is found to be equal to (V.sub.T /R.sub.B)1nK.sub.BA where the QB to QA emitter area ratio K.sub.BA has no significant dependence on V.sub.CC, T, or processing parameters. As a result, the output voltage V.sub.o is given by: EQU V.sub.o =R.sub.A (I.sub.CA +I.sub.CB)+V.sub.beA .perspectiveto.2(R.sub.A /R.sub.B)V.sub.T lnK.sub.BA +V.sub.T ln(I.sub.CA /I.sub.SA)(2)
Those skilled in the art will immediately appreciate that equation (2) is of the bandgap type with the first term having a positive, largely linear coefficient of temperature C.sub.T and the second term having a negative largely liner coefficient of temperature C.sub.T due to the strong dependence of I.sub.SA on T. By suitably choosing R.sub.A and R.sub.B (or the ratio thereof), V.sub.o can be made largely temperature independent. However, one disadvantage of the prior art circuit of FIG. 1 is that frequency-compensation circuitry must be used with the amplifier. Also, the use of PNP transistors is difficult to avoid if the amplifier is to operate efficiently.
Turning to FIG. 2, a current/voltage source prior art circuit is seen. Block 10 of FIG. 2 is essentially comprised of a cross coupled current stabilizer having transistors Q1, Q2, Q3, and Q4, with the collector-base junction of transistor Q1 being coupled to effectively form a diode, a resistor R1 connected between the voltage supply V.sub.CC and the collector of transistor Q1, and a resistor R3 coupled between ground and the emitters of transistor Q4. With the arrangement of block 10 which is described in detail in U.S. Pat. No. 3,930,172 to Dobkin, and with the balancing of the voltages, in accord with equation (1), the following is true: EQU R.sub.3 I.sub.C2 =V.sub.be2 +V.sub.be3 -V.sub.be1 -V.sub.be4 =V.sub.T ln(I.sub.S4 /I.sub.S2)+V.sub.T ln(I.sub.S1 /I.sub.S3) (3a)
With transistors Q1 and Q3 having equal emitter areas, V.sub.be3 .perspectiveto.V.sub.be1 due to the fact that substantially the identical current I.sub.C1 flows through both transistors Q1 and Q3. Hence, EQU R.sub.3 I.sub.C2 .perspectiveto.V.sub.T ln(I.sub.S4 /I.sub.S2)=(kT/g)lnK.sub.42 ( 3b)
where the Q4-to-Q2 emitter area ratio K.sub.42 is substantially independent of V.sub.CC, T, and processing parameters. Neglecting the small variation of R.sub.3 with T, I.sub.C2 is proportional to T but has substantially no dependence on the high voltage supply value V.sub.CC .
The addition of block 12 of FIG. 2 to block 10 provides a voltage reference in combination with a current source as might be suggested by Saul et al., "An 8-bit, 5 ns Monolithic D/A Converter Subsystem," IEEE JSSC, December 1980, pp. 1033-1039. While the provided arrangement substantially eliminates the temperature dependence of V.sub.o and uses only NPN transistors, V.sub.o is referenced to the position rail V.sub.CC and cannot be used in applications requiring that V.sub.o be referenced to the negative rail (often ground). A similar result (temperature compensated voltage reference circuit) is also found in U.S. Pat. No. 4,491,780 to Neidorff where the output voltage is also referenced to the positive rail.